Abstract

This collection of articles describes a new theory of glacial cycles and its application to a number of data sets that represent conditions during glacial times. The widely held conventional theory of glacial cycles, which is due to Milankovitch, attributes cycles in the earth's ice cover to perturbations in the motion of the earth and the resulting changes of insolation (solar heating) in the Northern Hemisphere. The strongest effects are expected to come from changes in the earth's obliquity (tilt of the earth's spin axis with respect to its orbit) and from the precession term that accounts for the delay between summer solstice (when the pole faces the sun) and perihelion (when the earth is closest to the sun). Perturbations of the earth's motion come from gravitational effects of the planets and the moon, and can be calculated with precision back at least 10 million years.

Another parameter describing the motion of the earth is the inclination i of the earth's orbital plane with respect to the invariable plane of the planetary systems. The invariable plane of the solar system is a plane perpendicular to the total angular momentum vector of the planets. Over the past one million years, the inclination has varied from about half a degree to about 3 degrees. During the time of low inclination, the earth accretes interplanetary dust at a greater rate than at times of high inclination. The dust particles, under the gravitational pull of the perturbing planets, tend to be concentrated in the invariable plane.

Dust particles can affect climate by altering the amount of solar radiation reaching the lower part of the atmosphere. At the high altitudes where the dust particles enter the atmosphere, the particles themselves can attenuate the incoming solar radiation, can sweep up water vapor which is a warming greenhouse gas, and can nucleate water particles to form high-altitude (noctilucent) clouds. The clouds would themselves reflect radiation.

While the detailed mechanisms of how astronomical dust can influence climate have not been completely worked out, this collection of articles shows that variation in inclination provides a better match for data sets on climate proxies than do variations of eccentricity. The theory also requires that density of dust in the vicinity of the variable plane varies with time. Beginning about one million years ago, the 100-kyr cycle became the dominating feature of variations in the total volume of ice covering the earth. Before that time, weaker variations are seen with 40-kyr and 20-kyr periods, consistent with variations in obliquity and precession.